1. Field of the Invention
The invention relates to a finger tester for the testing of non-componented printed circuit boards, together with a method for the testing of non-componented printed circuit boards using such a finger tester. In particular the invention relates to a finger tester with several test fingers, which are moved automatically to contact circuit board test points of a circuit board to be tested.
Testers for the testing of circuit boards may be divided basically into two groups, the group of finger testers (flying probe) and the group of parallel testers. Parallel testers are testers which, by means of an adapter, contact simultaneously all or at least the majority of the contact points of a circuit board to be tested. Finger testers are testers, for the testing of non-componented or componented printed circuit boards, which scan the individual contact points in sequence using two or more test fingers.
2. Description of the Prior Art
The test fingers are generally attached to a slide which may be moved along a cross-bar, while the cross-bar in turn is guided and able to move on guide rails. The slides may therefore be positioned at any desired point of a generally rectangular test array. To contact a contact point of a circuit board to be tested, either the slide is designed to move vertically on the cross-bar or the test probe is able to move vertically on the slide, so that the test finger may be placed on the contact point of the circuit board, the circuit board test point, from above and/or from below.
A finger tester is described in EP 0 468 153 A1 and a method for the testing of circuit boards using a finger tester is described in EP 0 853 242 A1.
Test probes for finger testers are disclosed in EP 1 451 594 B1, U.S. Pat. No. 6,384,614 B1, WO 03/096037 A1 and EP 0 990 912 A2.
EP 1 451 594 B1 discloses a test probe for a finger tester for the testing of circuit boards which does not have an independent drive. The test probe is provided with a test needle, the probe tip of which may be brought into contact with a circuit board test point. The test needle is fixed pivotably to a mount by at least two pairs of elastically sprung holding arms, wherein at least one of the holding arms is made of electrically conductive material and is electrically connected to the test needle. Viewed from above, the pairs of holding arms span a triangle so that the test probe, with a very low weight which is applied during contacting of the circuit board, nevertheless has high torsional stiffness to prevent the probe tip from slipping in the course of contacting.
WO 03/096037 A1 discloses a finger tester and a method of testing circuit boards with the aid of a finger tester, in which each test finger is assigned a camera which is focussed on the probe tip of a test needle, so that the camera may be used to detect any deviation of the probe tip relative to a circuit board test point to be contacted, and the position of the test finger in the plane of the circuit board may be suitably corrected. In this document the cameras are shown schematically in vertical alignment over the respective probe tip. In practice, though, these cameras are not arranged in vertical alignment over the probe tip but instead are offset slightly to the side, so that their direction of view of the probe tip encloses an angle to the vertical of approximately 30° to 50°. The cameras thus view the probe tip of the test needle from an angle. By this means it is reliably ensured that the test fingers do not collide with their cameras, even if they must simultaneously contact two circuit board test points lying very close together. This apparatus and the corresponding method for correcting the position of the probe tip has proved very successful in practice. The method may also be used to obtain calibration data which determine the localised relationship between the contact tips of the test needles and the physical circuit board test points of the circuit board to be tested.
It is also known to provide a finger tester with a camera which is separate from the test fingers and which, before the actual testing process in which the circuit board test points are contacted by the test needles, completely covers the surface of the circuit board. For this purpose the camera is moved like a test finger parallel to the surface. This camera is aligned so as to look vertically on to the surface of the circuit board. By this means a complete image of the surface of the circuit board is obtained. Since the camera is aligned to view the surface of the circuit board vertically, the recorded image of the surface of the circuit board has significantly fewer distortions than is possible with cameras mounted on the test fingers at an angle. A direction of view aligned vertically to the surface of the circuit board is desirable since in this way the quality of the image of the surface of the circuit board is much better, and a full image of the circuit board may be produced, which is much easier to process in subsequent optical analyses.
It would therefore be a great advantage if the camera could be arranged to detect the position of the probe tip of the test needle with a vertical direction of view relative to the circuit board, since then a finger tester could dispense with the additional camera for taking an overall image of the circuit board. This additional camera requires a separate drive mechanism which takes up precious space in the finger tester. Such an overall image of the circuit board could then be taken by a camera assigned to a test finger.
Known from EP 1 122 546 A2 is a tester for the testing of circuit boards. This tester is provided with an individual camera. The camera scans the surface of a test specimen by means of a mirror mounted at an angle. Light-emitting diodes are located adjacent to the mirror as light sources.